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JEE Main levelSingle correct optionDifficulty 2 of 3bm211

Four DNA samples have G + C content of 30%, 38%, 45% and 60%. Which sample has the highest melting temperature, and why?

  1. AThe 30% G + C sample, because A-T pairs have three hydrogen bonds
  2. BThe 60% G + C sample, because G-C pairs have three hydrogen bonds
  3. CThe 60% G + C sample, because G-C pairs have two hydrogen bonds
  4. DThe 45% G + C sample, because it has equal A-T and G-C pairs

Answer: B

More G-C pairs means more hydrogen bonds (three per pair) and stacking, so more energy is needed to separate the strands; the highest GC content gives the highest Tm.
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